In recent years, a multiple-input multiple-output (MIMO) system has attracted a great deal of attention as a technique of enabling high-capacity data communication. In the MIMO system, a radio transmission apparatus spatially multiplexes the transmission data (stream) and transmits the multiplexed transmission data through a plurality of transmission antennas, and a radio reception apparatus receives original transmission data by spatially demultiplexing reception data in which a plurality of transmission data are mixed on a propagation channel (for example, see Non-Patent Literature 1).
In the MIMO system, when Ntx is the number of transmission antenna ports and Nrx is the number of reception antenna ports, the number of streams that can be multiplexed is obtained by equation 1.[1]min(Ntx,Nrx)  (Equation 1)
Here, the function min(x, y) refers to a function that returns a smaller value between x and y. That is, the number of multiplexable streams is set to a smaller number between the number Ntx of transmission antenna ports and the number Nrx of reception antenna ports. Further, the number of actually multiplexable streams is decided according to the rank indication of a channel matrix of the transmission antenna ports and the reception antenna ports. As the channel correlation between the transmission antenna ports and the channel correlation between the reception antenna ports increase, the rank indication of the channel matrix decreases, whereas as the channel correlation between the transmission antenna ports and the channel correlation between the reception antenna ports decrease, the rank indication of the channel matrix increases.
As an extension system of the MIMO system, a multipath multiplexing system has been suggested that increases the number of streams by using multipath occurring on a propagation channel when the number of transmission antenna ports is larger than the number of reception antenna ports (for example, see Non-Patent Literature 2). Specifically, when Ntx is the number of transmission antenna ports, Nrx is the number of reception antenna ports, and L is the number of paths, the number of multiplexable streams is obtained by equation 2.[2]min(Ntx,Nrx×L)  (Equation 2)
Similarly to the MIMO system, even in the extension system of the MIMO system, the number of streams that can be multiplexed actually is decided according to the rank indication of the channel matrix of the transmission antenna ports and the reception antenna ports. That is, as the channel correlation between the transmission antenna ports and the channel correlation between the reception antenna ports increase, the rank indication of the channel matrix decreases, whereas as the channel correlation between the transmission antenna ports and the channel correlation between the reception antenna ports decrease, the rank indication of the channel matrix increases.